Don’t allow OPpTARGET_MY on postdec/inc

I was wrong in 9e319cc4f.  postfix ++/-- writes to its return value
before reading its argument.  If we optimise away the scalar
assignment in

    $a = $b++;

(that’s what OPpTARGET_MY does), then $a gets written to before $b is
read.  If $a and $b are the same, we get the wrong answer.  This bug
has been present under ‘use integer’ since 5.6.0.  I accidentally
extended it to non-integer ++/-- in 9e319cc4f.

(It’s not likely that someone will write $a = $b++, but it could hap-
pen inadvertently in more complex code.)